Classification. 39 
3 
adaptive, and resemblances as homological or of 
meaning in reference to a natural classification. To 
take another and more detailed instance, the Tas- 
manian wolf is an animal separated from true wolves 
in a natural system of classification. Yet its jaws and 
teeth bear a strong general resemblance to those of 
all the dog tribe, although there are differences of 
anatomical detail. In particular, while the dogs all 
have on each side of the upper jaw four pre-molars 
and two molars, the Tasmanian wolf has three pre- 
molars and four molars. Now there is no reason, so 
far as their common function of dealing with flesh is 
concerned, why the teeth of the Tasmanian wolf 
should not have resembled homologically as well as 
analogically the teeth of a true wolf; and therefore 
we cannot assign any intelligible reason why, if all the 
species of the dog genus were separately created with 
one pattern of teeth, the unallied Tasmanian wolf 
should have been furnished with what is practically 
the same pattern from a functional point of view, 
while differing from a structural point of view. But, 
of course, on the theory of descent with modifica- 
tion, we can well understand why similarities of 
habit should have led to similarities of structural 
appearance of an adaptive kind in different lines of 
descent, without there being any trace of such real or 
anatomical similarities as could possibly point to 
genetic relationship. 
Lastly, to adduce the only remaining argument 
from classification which I regard as of any consider- 
able weight, naturalists have found it necessary, while 
constructing their natural classifications, to set great 
store on what Mr. Darwin calls “ chains of affinities.” 
